Field of the Invention
The present invention relates to an image input system, and more particularly, it relates to an image input system which optically reads an image of an object and converts the same into digital signals to be supplied to an image processing system. Description of the Prior Art
In an image input system used as a front stage of an image processing system such as a facsimile or a process scanner, an image is generally photoelectrically read by a photoelectric converter such as a photomultiplier, an optical diode or a charge coupled device (CCD). However, as is well known in the art, intensity of light incident upon the photoelectric converter from the original image is expressed by an exponential function of optical density D of the image, and hence the levels of analog electric signals outputted from the photoelectric converter are also expressed by the exponential functions of the optical density D, and the signals are not proportional to the optical density of the image. Therefore, the analog output signals from the photoelectric converter have generally been logarithmically transformed by means of such as an analog operational amplifier to obtain output signals which are in linear relation to the optical density of the original image, in order to simplify the processing in the image processing system and to improve gradation reproducibility. A circuit employing such an analog operational amplifier is relatively simple in construction, because it can be formed by one operational amplifier and a diode when an operational amplifier (so-called LOG amplifier) performing the said logarithmic transformation is utilized.
However, an analog operational amplifier formed by a semiconductor integrated circuit is inferior in frequency response and is not appropriate to high-speed processing. Further, the characteristics of the operational amplifier are generally unstable by temperature drift etc., and hence the gradation cannot be correctly reproduced. In addition, such a circuit employing the analog element cannot be easily adjusted, leading to inferior productivity.